Method of treating magnesium



Jan, 27, 1942. c. E. NELSON METHOD OF TREATING MAGNESIUM Filed May ll, 1940 ATTORNEYS Patented Jan; 27, 1942 METHOD OF TREATING MAGNESIUM Charles E. Nelson, M-dland. Mich., assigor to The Dow Chemical Company, Midland,`Mich., a

corporation of Michigan Application May 11, 1940, Serial No. 334593 7 Claims.

This invention relates to the treatment of magnesium. It more particularly concerns a method of treating commercial magnesium whereby it is rendered highly pure, or the magnesium in magnesium scrap, such as castings, alloys and the like, can be recovered in a highly pure state. e

I have found that the presence of certain impurities in magnesium, even though these be present in very limited amounts, greatly accelerates the rate at -which magnesium corrodes. For example, it is recognized that the presence of a small amount of iron in magnesium gre'atly accelerates the rate at which magnesium corrodes and may thus render it unsuitable for many' commercial uses. The impurities nickel and copper affect the rate at which magnesium metal corrodes similarly.

. To remove these impurities ordinarily present in the magnesium produced by 'presentpractices, it has heretofore been proposed to subject the magnesium to sublimation or distillation. However, I have observed that unless the metal being puried is subjected to repeated sublimation' or distillation, as the case may be, the imhereinafter fully described and particularly V pointed out in the claims, the accompanying purities are not generally removed to the extent that they no longer greatly accelerate the rate at which magnesium corrodes or otherwise adversely afiect the properties of the' metal. In some instances, even by repeated distillations or sublimations, the metal cannot be rendered satisfactorily pure.

It is, therefore, the principal object of the invention to provide a method of rendering magnesium so highly pure that the impuritiesno longer have substantially any adverse efiect as regards the rate at which magnesium corrodes.

Another object of the invention is to provide a method whereby magnesium can be rendered pure in a' single operation. Other objects and advantages will be apparent as the description proceeds.

I have discovered that, if the magnesium to be purifled is caused to vaporlze in the presence of a metal selected from the group of metals consisting of lead, tin, bismuth, calcium, silicon, and antimony and thereafter passed through a filter bed of a refractory material selected from the group of refractories comprising the oxides and silicates of metals located above managanese in the electromotive force series of metals, impurities can be removed to the extent that the rate to which commercial magnesium corrodes is greatly reduced. v

The invention then resides in the method drawing and following description setting Iorth in detail a mode 'of carrying out the inventon. such mode illustrating, howev er butone oi' various ways in which the principle or the invention may be used.

In said drawing:

Fig. 1 is a sectional view in elevation of the apparatus;

Fig. 2 is a top plan View of said apparatus.

In the drawing an upright cyundrica iron 'etort I is shown closed at the bottom and is provided at the top end with a fiange 2, to which is attached a gasketsealed head 3 by means of suitable bolts 4. Condenser 5', in the form of a section of iron pipe closed at its lower end is* attached centrally to the interior side of head 3 and extends into the retort. Exteriorly of the denser 5. A mass 'of purified condensed magnesium 8 is shown clinging to the con'denser 5.

A vacuum connection 9 communicates through head 3 with the interior of the retort and ;provides a means whereby the retort may be connected to a vacuum pump (not shown) and held under reduced pressure. A cooling coil ll! intimately attached to head 3 (as by welding) encircles the head directly above gasket ll and serves to prevent the gasket from becoming over-heated. Interiorlyxof the retort a removable iron charge container |2 partially filled with a solid charge |3 oon sisting of a quantity of impure magnesium and a suitable metal added .to effect the purification of the magnesium rests on the bottom of the retort. A shallow perfo-.

rate metal basket M, which engages the top rim of container z, is Suspended from charge con-- tainer |2 by means of fiange |5.' The perforate basket is shown filled with granular particles of a re'actory material, designated generally by numeral IS, which acts as a filter through which the vaporized magnesium passes. A perforate cover H is provided for basket !4 and serves to confine the filter bed during the purifying operation. As a means of heating the retort electrical leads |8 are connected to a resistance wire !9 which encircles the lower portion of the retort. A suitable insulating material 20 surrounds the retort and serves to protect it from excessive heat loss due to radiation'.

In carrying out the method of the invention the iron charge container z, filled with charge !3, ,consisting of impure magnesium and the tort of gases to line 9. When the desired degree of Vacuum is obtained electrical current is supplied to the resistance wire s through electrical leads |8 to heat the retort, care being taken in the meantime to maintain the vacuum. The

impure magnesium as well as the added metal rapidly melts' and as the magnesium vapor forms it passes through filter bed of refractory material I& which exerts a further purifying action and finally collects and condenses on the cold surface of condenser where it adheres in the form of compact crystals. After such time has elapsed as will indicate that substantially all the magnesium has been distilled off the current may be shut ofi, the vacuum broken and the fianged head 3 detached from the retort.- The magnesium crystals can then be scraped from the condenser. Additional impure magnesium may be immediately added to the charge container without removing the residue therefrom and the process of purifying the metal continued. Howmost effective puriflcation. As afore-mentioned it is generally desirable to renew the filter bed after each run since the materials tend to become soft and crumble easily after being heated to the temperature required for the operation of the process.

The purifying efiect produced by passing the" vaporized metal through a bed of these refractories is not entirely' due to a. mechanical filtering action, but is also due to some chemical action between the 'impurities and the refractory. This is evidenced by the fact that by merely adding a quantity of rock wool to a charge of impure magnesium, and thereafter distilling or subliming the magnesium, a product of greater purity is obtained than can be obtained by just distilling impure magnesium. Thus merely bringing the vaporized magnesium into contact with the filtering or puriying .medium suflices to yield a puriever, it is generally preferable to renew the `rea fractory contained in the perforate metal basket after each purifying operation since it tends to crumble and become powdery under the temperature conditions of operation.

As afore-mentioned, the metals suitable for use are the metals selected from the group of metals consisting of lead, tin, bisrnuth, calcium, silicon, and antimony. If desired, mixtures of these metals may be suitably employed. For example. a particularly effective mixture consists of tin and lead. In general the' amount of such metal ,refractories consisting of oxides and silicates located above manganese in the lectromotive force series of metals. Examples of such refractories are calcium silicate, barium oxide, aluminum oxide and the like. If desired mixtures of these metals may be employed. Complex mixtures oi oxides and silicates have also been found to be highly eflective. For example,. a material preferred because of its especial eflectiveness is the ordinary rock wool or mineral wool of commerce which is obtained by mechanically processing naturally occurring rock compositions consisting mainly of silicon oxide, alumina, and magnesia. Another silicate useful forhe purpose at hand is exploded mica; The materials are preferably employed in the form;f g'anular particles or, in the case of rock wofl, as 'fibrous material, and in this form give a filter bed that is readily permeable to the' magnes`am' vapor. The filter bed made of these materials should be made of /z to 2 inches or more in thickness to produce the fied condensate.

It is usually desirable to operate under relatively high vacuum, as for example, less than 2 millimeter-s of mercury, and preferably below 0.2 of a millimeter, since under such highly reduced pressure the purified magnesium condenses out in the form of a dense compact mass rather than in the form of long needle-like crystals. Condensation in a dense compact form is desirable because relatively little surface area is exposed to oxidation when the metal is later rehandled for remelting or the like, in comparison with the area exposed when a, needle-like crystal structure is produced.` Operating at highly reduced pressure ofiers the additional advantage that the temperature required to eifectively carry out the process is such that the apparatus may be constructed of readly available iron or steel without any danger of destruction due to the high temperature necessary if the purifying operation is carried out at higher pressure. If desired, a suitable inert gas such as helium, argon, and the like may be introduced into the retort to prevent destructive attackby air or other gases upon the purified metal, although under the preferred vacuum conditions such procedure is umecessary.

The temperature to be employed in the distillation or sublimation may, of course, be varied quite widely, as for example, between about 500 to 800 C., depending, however, upon the degree of vacuurn employed. At 0.05 millimeter pressure of mercury a temperature of 700 C. has been foundhighly satisfactory. It is preferable that the magnesium be c'aused to vaporize at such a temperature that the added metal melts in the presence of the magnesium vapor.

The time required to purity a batch of metal also depends upon the amount of beat supplied to the metal as well as the pressure employed. In a retort such as illustrated, 16 inches in diameter, it is possible to distill oil' from three to four pounds of magnesium per hour by holding the 'temperature at about 700 C. and the pressure at 0.05 millimeter.

A'generally desirable combination to employ, because of the high degree of purity oi' the product obtained, is from 15 to 20 per cent by weight of lead in combination'with a filter composed of rock wool.

,This application is a continuation-in-part of my application serial No. 257,924, filed February 23, 1939. Other modes of applying the principle of my nvention may be employed instead of those explained, change being made as regards the method herein dlsclosed, provided the step or steps tated selected from the group of metals consisting of by any of the following claims'or the equivalent of such stated step or steps be .employeii. i I therefore particularly point out and distinctly claim as my invention: 4 v

1. The method of prodicingsubstantially pure v magnesium comprising introducing' impure magnesium into direct contact with from to 50 per cent by Weight of a metal selected-from the group of metals consisting of lead, tin,' calciun', bismuth, antimony, and silicon, applying heat so as' to cause the metals to alloy, continuing the'application of heat so as to cause the. magnesium to vaporize, and thereafter passing the magnesium vapor through a filter bed of a refractory material selected from the group of refractory materials consisting of the oxides and silicates of the metals located above manganese in 'the electromotive series of metals. v

2. Themethod of producing substantially pure magnesium comprising bringing .impure magnesium into direct contact with from 25 to 35 per cent by weight of a metal selected from the group of metals consisting of lead, tin. calcium, bismuth, antimony, and silicon, applying heat so as to cause the metals to alloy, continuing the application of heat so as to vaporize the magnesium while maintaining the metals under reduced pressure, and thereafter passing magnesium vapor through a filter bed comprising a refractory matev rial selected from the group of refractory materials consisting of the oxides and silicates of the metalslocated above manganese'. in the electromotive series of metals. p

3. The method of produeing substantially pure magnesium comprising heating an alloy consist'- 3 ing of magnesium and from '10 to 50 per cent by weight of at least one of the metals selected from the group of metals consisting of lead, tin, calcium, bismuth, `antimony, and silicon so as to cause the magnesium contained in the alloy to vaporize, passing the magnesium vapor through a filter bed of a refractory material selected from the group of refractory materials consisting oi' the oxides and silicates of'the'metals located above manganese in the electromotive series of-metals.

4. The method of producing substantially pure magnesium comprising heating an alloy under reduced pressure consisting of magnesium and from 10 to opercent of at least one of the metals lead, tin, calcium, bismuth, antimony, and silicon so as to cause the magnesium contained in the alloy to vaporize, passing the magnesium vapor through a filter bed of a refractory materialselected from the group of refractory materials 'consisting of the oxides and silicates of the metals located above manganese in the electromotive.

series of metals.

5. The method of producng substantially pure magnesium comprising heating an alloy consisting otmagnesium and from 10 to per cent by weight of at least one of the metals selected from the group of metals consisting of lead, tin, caleium, bismuth, antimony, and silicon so asto cause the magnesium contained in the alloy to vaporze and thereafter bringingthe magnesium vapor into contact with a refractory material selected from the group of refractory materials consisting'of the oxides 'and silicates of the metals located above manganese in the electromotive series of metals without causing the magnesium vapor to condense.

6. The method of producng substantially puremagnesium comprising heating an alloy consisting. of magnesium and from 25 to 35 per cent by y weight of at least one of the metals selected from the group of 'metals consisting of lead, tin, calcium, bismuth,' antimony, .and silicon so as to cause the magnesium contained in the alloy to vaporize, bringing the magnesium vapor into contact with a refractory material selected from the group of refractory materials consisting of the oxides. and silicates of the metals located above manganese in the electromotive series of metals without condensing said magnesium vapor, and thereafter separately condensing the' magnesium vapor.

7. The method of producing substantiall'y pure magnesium comprising bringing impure magnesium into' direct contact with from 25 to 35 per cent by weight of lead, applying heat so as to alloy 'the magnesium and lead, continuing the application of heat so as to vaporize the magnesium 

